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Commentary |
Regional Medical Physics Department, Newcastle General Hospital, Westgate Road, Newcastle-upon-Tyne NE4 6BE, UK
In 2001 the Institute of Physical Sciences in Medicine published a report [1] on cost-effective methods of dose reduction in medicine. Options were compared by assigning a monetary value to radiation risk enabling a comparison to be made between a given radiation detriment and the cost of implementing a measure that would reduce it. While Moores [2] has argued that large dose saving costs (£600 million per annum) may be justifiable irrespective of the actual doses received, Williams [3] has suggested that risks resulting from some lower dose procedures might be regarded as negligible. More recently, Clarke [4] has indicated that existing international recommendations on dose reduction may be too closely linked to formal cost–benefit analysis.
Clarke [4] has summarized the current intentions of the International Commission on Radiological Protection (ICRP) with regard to occupational and public exposure. Optimization of individual exposure, particularly that of the most exposed group of persons, is preferred over measures to limit collective dose. While patient exposure is not specifically targeted, the approach may nonetheless be relevant in any such review.
At a meeting at the British Institute of Radiology (BIR) in 2004 [5] aspects of dose reduction in diagnostic radiology were considered. Areas identified with potential for further optimization were
Certain of these measures, notably digital imaging, may be seen to encompass broader policy issues, while others may have no direct cost. An existing optimization method, rooted in detriment and monetary value, may therefore be an inappropriate tool to optimize radiation risk and clinical utility. In diagnostic radiology, although individual patient dose (and therefore risk) may be relatively low, a number of other factors, such as image quality, ease of use and patient throughput may contribute to a decision. It may be appropriate therefore to develop an optimization tool that ranks options in accordance with accepted indications.
To be consistent with ICRP proposals [4] this ranking must, as a minimum, reflect the change in dose resulting from the proposed change in practice ("Dose Change"), the effective dose to the most exposed group ("Dose") and the number of persons exposed ("Exam Frequency"). Certain other ICRP factors, such as "Age at Exposure" may also impact on the ranking. However to be both credible and transparent, the method must have scope to accommodate factors such as ease of use, image quality, patient throughput and cost. Since not all elements would be relevant in every instance, the ability to customize an approach would be important.
In the following paragraphs, one such ranking method is outlined in general terms. It is stressed that although it holds to the general principles outlined above, it is presented as an example appropriate to the situation considered and is not intended to be prescriptive. In particular, the scoring used may not necessarily be appropriate in all situations.
The situation modelled is one in which a routine review of radiographic technique and procedure is undertaken, but where no significant investment or cost savings are sought. In order to reflect the routine and localized nature of the review, options are further restricted to those that may be associated with minimal change in image quality or diagnostic potential. Options were selected from those considered at the BIR meeting [5] and detailed in the above list, although in reality it is likely that some of these measures would have been already implemented, at least in part. Factors selected for inclusion were those seen to be relevant to the outcome, i.e. dose and risk parameters, but not image quality or other patient related factors. A monetary parameter was included to incorporate minor cost changes.
The suggested scoring system is shown in Table 1
. In this example, each factor scored has been allocated three or four levels, with scores ranging from 0 to 3 accordingly. Levels for each factor have been tied to specific ranges in numerical value for that factor. For added clarity each level has also, where possible, been associated with a language descriptor, e.g. "negligible".
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for Dose is in accordance with the above bandings, except that levels have been modified to take account of the overall range in patient effective dose across diagnostic radiology. This has resulted in a four point scoring range that reflects a logarithmic scale of values.
Language descriptors and associated numerical ranges have already been established for changes to patient dose parameters in diagnostic radiology where those changes have resulted from equipment related factors monitored through quality assurance testing [6]. A remedial level (15%) and suspension level (30%) have been specified for reproducibility in the radiation output of an X-ray tube and generator. Levels were set to reflect usual levels of machine performance, and by implication the technical limitations on routine control of patient dose. The scoring indicated in Table 1 in relation to Dose Change incorporates the above levels while adhering to a four point scoring range that follows a logarithmic scale of values. The logarithmic scale is finer than that used for Dose, however, reflecting an emphasis on proactive control of patient dose.
Risk resulting from radiation exposure has already been classified according to age at exposure [7]. Fatal lifetime cancer risks of 11.1% per Sv, 4.2% per Sv and 1.6% per Sv have been quoted for age ranges at exposure of 0–9 years, 40–49 years and 70–79 years, respectively, although these figures incorporate large individual uncertainties. The scoring indicated in Table 1 in relation to Age at Exposure recognizes three age ranges: Geriatric (70+ years), General Adult Population (20–69 years) and Paediatric (0–19 years). In accordance with the above factors, the scoring reflects a trend in the values (i.e. risk) that is approximately logarithmic in nature.
Similarly, for "Exam Frequency" the scoring system indicated in Table 1 is set to reflect a logarithmic scale of values that spans the range observed in diagnostic radiology. In the case of "Cost Change" a similar scoring system has been adopted in Table 1 for this relatively simple situation, although in other cases it may be possible to incorporate other indicators such as budgetary tolerance, or some other aspect of corporate fiscal policy. In this case, changes might be in either direction, implying a positive (i.e. cost benefit) or negative (i.e. cost detriment) score.
Depending on the option appraisal being carried out, further factors could be incorporated. Many changes to imaging practice may result in either enhanced or impaired diagnostic information. Such changes, (e.g. introduction of virtual colonoscopy replacing barium enema) are unlikely to be characterized by simple measures of image quality, but could in principle provide input to a formal option appraisal exercise through semi-quantitative measures of diagnostic acceptability. For common X-ray examinations some criteria have already been determined [8, 9] based on the visibility (or otherwise) of certain anatomical or radiographic features. For other applications it may be that locally devised criteria would suffice.
Examples of the application of the scoring system indicated above to various dose reduction options are shown in Table 2. Scores have been added arithmetically to obtain a total. Total scores have been compared in order to rank the options, with the highest score representing the most favourable option. The conclusion for these particular examples is that improved scan protocol selection in paediatric CT of the abdomen would be a priority.
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Even given general acceptance of the scoring and the factors used, the reliability of the information resulting from the model, and therefore the significance of any individual result, depends critically upon the robustness of the data input. Therefore the individual results shown in Table 2 are not necessarily definitive. However the techniques discussed represent a possible framework to develop priorities on a local basis.
The advantage of the suggested system is that it retains simplicity and flexibility and, unlike existing proposals, is not tied exclusively to monetary costs and cost equivalents. It is essentially a simple "tick list" which incorporates some qualitative information. If used sensitively, it should give preferential encouragement to those developments and dose saving proposals that are seen to be most beneficial.
Acknowledgments
A number of individuals have provided valuable comments during the drafting of this paper. These include Robert Corbett, Philip Dendy, Roger Harrison, John Kotre, Andy Rogers, David Sutton and Martin Vosper.
Received for publication August 2, 2004. Revision received April 21, 2005. Accepted for publication May 3, 2005.
References
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